1. Field of the Invention
The present invention relates generally to acoustic surface wave energy processors employing acoustical means for processing electrical input signals and yielding modified output electrical signals and, more particularly, concerns surface acoustic wave reflectors with cooperating weighted arrays of segmented and non-segmented lineal elements.
2. Description of the Prior Art
For application in acoustic wave filters, resonators, and the like, suitable acoustic wave reflectors have long been recognized as of significant importance. Weighting has been employed in such reflector devices to produce predetermined reflectivity versus-frequency characteristics. Two types of such reflectors are in general use: 180.degree. reflectors and 90.degree. reflectors, the 180.degree. reflectors being often applied in resonators and the 90.degree. devices in pulse compression and frequency sorting devices and other filters.
As is discussed by W. J. Tanski in the technical paper "SAW Responators Utilizing Withdrawal Weighted Reflectors", I.E.E.E. Transactions on Sonics and Ultrasonics, November 1979, page 404, considerable use has been made of uniformly distributed reflector arrays in such applications. Being uniform, each reflector element in the array has the same length perpendicular to the direction of acoustic energy flow and the same acoustic reflectivity. But the reflection characteristic of a uniform reflector array has relatively strong undesired side lobes, resulting in undesirable resonant modes in surface acoustic wave resonators employing them. Such modes manifest their presence in off-resonance ripple, degrading out-of-band rejection. Further, the reflection phase in a uniformly distributed reflector array has an undesirably steep slope.
Weighting of the individual reflectors of an array may be used to reduce side lobe levels and the slope of the reflection phase. This may be achieved by varying the lengths of each reflector element or by otherwise varying the reflectivity of each reflecting element.
It has also been recognized that useful reflectors may be made by the selective omission of certain reflecting elements. Such omission or withdrawal of elements from the array has proven to reduce reflection side lobes and to eliminate possible distortions due to length weighting. They are evidently simpler to construct, having fewer elements per array. Weighted reflector arrays have also been achieved by replacing the omitted element by a pair of narrow (split) finger elements, by replacing omitted adjacent pairs of elements by a broad finger element, or by relative displacement of reflecting elements. Each kind of omission or withdrawal weighting scheme is effective to a degree; however, the relative coarseness of these techniques make high manufacturing precision impossible.
Dot arrays provide a highly effective way of achieving precise weighting, as described in the L. P. Solie U.S. Pat. No. 4,055,820 for a "Reflective Dot Array for Acoustic Wave Processing", issued Oct. 25, 1977 and assigned to Sperry Corporation. Instead of lineal reflectors, there is employed an array of lines of discrete scattering points, each point with dimensions less than a wave length and disposed at the substrate surface to reflect the incident acoustic wave. While greater freedom from distortion of the response caused by multiple reflections between etched grooves is attained, the dot array concept finds special purpose application primarily where its relatively high cost may be tolerated. Also, withdrawal weighting has generally not demonstrated as high a suppression of side lobes as theoretically predicted or as can be achieved using dot arrays.